Mechanisms of nuclear pore complex assembly - two different ways of building one molecular machine

Abstract

The nuclear pore complex (NPC) mediates all macromolecular transport across the nuclear envelope. In higher eukaryotes that have an open mitosis, NPCs assemble at two points in the cell cycle: during nuclear assembly in late mitosis and during nuclear growth in interphase. How the NPC, the largest nonpolymeric protein complex in eukaryotic cells, self-assembles inside cells remained unclear. Recent studies have started to uncover the assembly process, and evidence has been accumulating that postmitotic and interphase NPC assembly use fundamentally different mechanisms; the duration, structural intermediates, and regulation by molecular players are different and different types of membrane deformation are involved. In this Review, we summarize the current understanding of these two modes of NPC assembly and discuss the structural and regulatory steps that might drive the assembly processes. We furthermore integrate understanding of NPC assembly with the mechanisms for rapid nuclear growth in embryos and, finally, speculate on the evolutionary origin of the NPC implied by the presence of two distinct assembly mechanisms.

Keywords: cell cycle; mitosis; nuclear assembly.

Figure 1

Nuclear pore assembly during cell cycle. During anaphase and telophase, the nuclear membranes that are absorbed in the endoplasmic reticulum (ER, blue) at metaphase assemble on chromatin (blue gradient) together with the nuclear pore complexes (NPCs, orange). The nuclear envelope (NE) assembly is delayed in the chromosome regions next to the spindle pole and the central spindle area (called ‘core region’) due to dense spindle microtubules (red lines) on the DNA surface. During nuclear growth after telophase, the NPCs assemble de novo into the double membrane barrier of the NE.

Figure 2

Models for the NE assembly during chromosome segregation in anaphase. The nuclear membranes will be supplied by the lateral expansion of the ER (blue) on the chromatin surface (blue gradient) and/or by the cisternal stacking of the ER.

Figure 3

Two distinct assembly mechanisms of the NPC. (A) Postmitotic NPC assembly proceeds by a radial dilation of small membrane openings 24. The initial prepore contains dense material in the center of the membrane hole as well as the nuclear ring. It dilates radially and obtains active transport competence during the inner ring and the cytoplasmic ring complex assembly. The central channel continues to maturate even after the membrane dilation stops, and the NPC gradually obtains permeability barrier function against small proteins. Molecular players involved in individual steps of the assembly are indicated below. Possible molecular requirements are shown in gray. ONM, outer nuclear membrane; INM, inner nuclear membrane. (B) Interphase NPC assembly proceeds by an inside‐out extrusion of the nuclear membrane 27 . The intermediate is a dome‐shaped evagination of the INM, that contains the nuclear ring structure underneath the INM from the beginning and grows in size vertically and laterally until it fuses with the flat ONM. Possible molecular players in individual steps of the assembly are shown as in (A).

Figure 4

Two theories for the evolution of the eukaryotic endomembrane system. (A) ‘Outside‐in’ model. The surface of ancestral eukaryotic cells invaginated and branched progressively into a cytoplasmic membrane network. The function of NPC components (pink) would have been a passive stabilization of regions with high membrane curvature to prevent closure of the membrane cisternae around DNA (dark and light blue dots). (B) ‘Inside‐out’ model. Membrane filopodia protruded progressively through the glycocalyx to engulf extracellular material and thereby create new cytoplasm while the original cell becomes the nucleus. The function of NPC components would have been to actively promote membrane evaginations. Modified from 103. Arrows indicate direction of cytoplasmic flow.